Over the past two decades, the accumulated literature suggests that periconceptional folic acid supplementation can reduce both the occurrence and recurrence of orofacial clefts and neural tube defects in humans by about 50%. This has major public health implications, as about 10,000 pregnancies per year in the United States are complicated by either neural tube defects or orofacial clefts. These malformations are among the most common of all human birth defects, yet their etiologic basis and underlying embryology remain poorly understood. The epidemiological evidence suggests that the protective effects of folic acid are unlikely to simply be maternal folate deficiency. Rather it may be deficient fetal folate metabolism, as well as the teratogenic effects of the elevated homocysteine levels that accompany folate deficiency. The applicant has developed a transgenic "knockout" mouse model lacking functional folate binding protein (FBP-1 and FBP-2). Homozygous null embryos for the FBP-1 have lethal neural tube defects. The general hypothesis is that an abnormal maternal and/or fetal folate receptor increases the risk for orofacial clefts and/or neural tube defects due to an inability to adequately bind and transport folate to both the oral and neural epithelia of the developing embryo. Experiments are proposed to determine the function of the FBPs during palatal and neural tube closure in the offspring of dams whose genes have been inactivated by homologous recombination in embryonic stem cells. The experimental model uses manipulation of folate and homocysteine levels of the dams and embryos by genetic or dietary means, with a number of morphological, molecular and biochemical endpoints. The conclusion of these studies have the potential to lead to a greater mechanistic appreciation for the protective effect of folic acid supplementation on embryonic development, the relative importance of folate and homocysteine metabolites and a better understanding regarding the role of cellular proliferation and/or cell death in craniofacial and neural tube development under conditions of variable folate and homocysteine availability.